Supported CoMo catalysts have been used for decades in the fixed bed hydrotreatment of diesel fuels. These catalysts have been proven to be very effective for removing the bulk of the sulfur content from diesel fuels at relatively low cost. Moreover, until the 2006 U.S. regulatory changes reduced the maximum sulfur levels in diesel fuels from 500 ppm by weight (S500) to 15 ppm by weight (S15), these catalysts were also typically very robust, generally lasting for years before replacements were necessary, and thereby typically affording very low catalyst cost.
However, delivering S15 generally requires an increase of the severity of the hydrotreating conditions when using the same catalyst, which in turn leads to faster catalyst deactivation, particularly in relatively low-pressure units that have to rely more on increased temperature to produce S15. The catalyst deactivation often accelerates so much that it significantly affects the cost of refining diesel fuel. There is, therefore, a need for process and/or catalyst solutions to mitigate the accelerated catalyst deactivation involved in hydrotreating diesel fuels to sulfur levels below 15 wppm.
According to the present invention, it has now been found that the impact of catalyst deactivation, particularly during the start up, can be reduced by using a start-up feed with reduced aromatic content as compared with that of the feed to be hydrotreated.
U.S. Pat. No. 3,436,338 discloses that hydrocracking catalysts which have been partially deactivated by polycyclic aromatic hydrocarbons present in the charge stock are reactivated by introducing a feed having a lower polycyclic aromatic content. This patent is, however, silent as to the effect of lowering the overall aromatic content of the deactivation rate of a fresh, undeactivated catalyst.